The invention relates to a monolithically integrated semiconductor circuit with at least one lateral transistor whose base zone exhibiting one conductivity type is delimited from its environment in the semiconductor crystal exhibiting the opposite conductivity type by means of a pn junction in the non-conducting direction via a d.c. voltage.
In order to achieve a sufficient decoupling of the semiconductor elements comprising the circuit in monolithically integrated semiconductor circuits, it is usual to form the individual semiconductor elements of the circuit--in so far as this is required,--in a respective insulation trough. The term "insulation trough," means a semiconductor area of the one conductivity type which is embedded in a semiconductor area of the opposite conductivity type upon formation of a so-called "insulating pn junction."
In the manufacture of such semiconductor devices, one proceeds from a monocrystalline semiconductor slice of the one conductivity type consisting, particularly, of silicon as the substrate on whose one surface side a monocrystalline layer of the same semiconductor material, but of the opposite conductivity type, is epitaxially deposited. By means of masked diffusion and/or implantation, an epitaxial layer is then provided with a plurality of strip-shaped insulation zones exhibiting the conductivity type of the substrate, which insulation zones receive contact over their entire length with the substrate and form an inter-connected zone of the one conductivity type together with this, in which the parts of the epitaxial layer still exhibiting the opposite conductivity type are embedded as islands which are respectively provided as an insulation trough in the monolithically integrated semiconductor circuit to be finished in the usual manner. In most cases, each insulation trough contains at least one semiconductor element, for example, a transistor. The transistor, for example, can be designed as a pnp lateral transistor whereby the emitter-base pn junction and the collector-base pn junction are arranged next to one another. The insulation trough or, respectively, the part of the insulation trough not covered by the manufacture of the emitter zone and collector zone of the lateral transistor forms the base zone of the transistor which is delimited from the area of the semiconductor crystal, above all the substrate, lying outside of the insulation trough and not belonging to the transistor by means of a pn junction which is reverse biased during operation of the integrated circuit.
However, in the use of lateral transistor structures, it is observed that they always divide their emitter current J.sub.E into a lateral active component J.sub.EL and a vertical parasitic component J.sub.EV. The parasitic component J.sub.EV divides into a recombination component (base current) and into a second component J.sub.s which flows off into the substrate, i.e. into the area outside of the insulation trough.
On its way to the next metallic substrate contact, this substrate current generates a voltage drop with parasitic effects, for example, by changing the bias of insulating pn junctions into the forward direction, by forming thyristors, etc., and, in the extreme case, causes the destruction of the integrated circuit containing the lateral transistor.
The relationships become particularly unfavorable upon saturation operation. Namely, in the case of the design of the lateral transistor as a pnp transistor, as soon as the collector potential has dropped so far that the base-collector pn junction is forward biased, the collector in turn begins to inject into the base and becomes a second emitter whereas the substrate becomes a collector. Thus, substrate currents with the said injurious effects for the operation and the existence of the integrated semiconductor circuit again flow. Only in this case, the current gain of the parasitic pnp transistor which has arisen is considerable because of the small base width, the great emitter edge length and the lack of a screening intermediate electrode.